1. Field of the Invention
The present invention relates to a display data generating device that generates data for displaying an image on a display device or the like.
2. Description of the Related Art
A car navigation system, a game machine, a cellular phone, and so on internally have a display data generating device for generating display data to be displayed on a screen.
This display data generating device has a memory device to which memory areas corresponding to pixel areas of a display screen are allotted. An SDRAM (Synchronous Dynamic Random Access Memory) that operates in synchronization with a clock, or the like is used for the memory device. The display data generating device receives pixel coordinates and pixel information outputted from a controller, converts the pixel coordinates to an address in the memory device, and according to the pixel information, modifies pixel data stored in a memory area designated by the address obtained by the conversion.
Generally, in a display data generating device, a plurality of memory devices are connected in parallel and the bus width (one word) of a data signal of each memory device is constituted of 64 bits or 128 bits. Pixel information necessary to constitute one pixel is normally 16 bits or 32 bits. For example, when information of one pixel is constituted of 16 bits and the bus width of a memory device is constituted of 64 bits, one access to the memory device enables pixel data read or write for 4 pixels. Reading or writing pixel data for one word at a time from/to the memory device improves access efficiency of the memory device (disclosed in, for example, Japanese Unexamined Patent Application Publication No. Hei 6-119437).
The conventional display data generating device executes read and write operations to the memory device for every one word, however, there has been a demand for further improvement in access efficiency in order to increase image display speed. Further, the display data generating device modifies the pixel data for every one word as described above. This makes it necessary to control circuit blocks in the display data generating device every time an access for one word occurs, which requires constant supply of clocks to each of the circuit blocks. As a result, it has been difficult to reduce its power consumption.